It is well known that phosphorus (P) has a deleterious effect in iron and steel, especially in stainless steels it causes and develops hot cracking, stress corrosion cracking, etc. Therefore, it is desirable to reduce phosphorus content as much as possible. Until today, however, it has been considered impossible to dephosphorize molten iron containing not less than 3% of Cr.
In the known techniques, the phosphorus involved in the raw materials remains in the resulting products, the amount of which is about 300 ppm. If production of stainless steels and phosphorus content of which is less than this amount is intended, there is not means other than using carefully selected low phosphorus content materials, which, of course, results in high price of the products.
Rather recently, it has been proposed for the purpose of dephosphorization of molten pig iron to incorporate oxides, carbonates or chlorides of alkali metals in the smelting slag. For instance, in Japanese Laying-Open Patent Publication No. 2322/78, "a dephosphorization agent to be used for dephosphorizing molten pig iron comprising a mixture of lime, iron ore, soda ash and fluorite, characterized in that iron oxide is added in an amount not less than 2.5 times the weight of the oxide or carbonate of an alkali metal, the ingredients are mixed and pulverized and heated at 600.degree. C. or higher so that compounds of iron oxides and alkali metal oxides are formed, and CaO is added in an amount from equal with to 10 times the amount of said compounds" is disclosed. In Japanese Laying-Open Patent Publication No. 26715/78, "an auxiliary refining agent for molten iron containing an alkali metal compound, to which a SiO.sub.2 -containing material containing not less than 50% SiO.sub.2 and/or a SiO.sub.2 -containing material in which the total content of SiO.sub.2, Na.sub.2 O, MnO and FeO is not less than 60% is added, whereby the amount of SiO.sub.2 and the SiO.sub.2 -containing material is respectively 20% or less and 50% or less" is disclosed. Further in Japanese Laying-Open Patent Publication No. 28511/78, "a dephosphorization, desulfurization or dephosphorization-desulfurization slag comprising 30-70% CaO, 10-40% Ca.sub.2 as the principal ingredients, and 1-30% of at least one of Na.sub.2 O, B.sub.2 O.sub.5, Na.sub.2 B.sub.4 O.sub.7, K.sub.2 O, Li.sub.2 O, NaCl, KCl and LiCl" is disclosed.
Although all these slags or refinining agents may be effective for plain pig iron, they are quite ineffective for dephosphorization of the Cr pig iron. All the descriptions of these three quoted Japanese Laying-Open Patent Publications relate dephosphorization of plain pig iron and there is no reference to dephosphorization of the Cr pig iron.
Difficulty of dephosphorization of the Cr pig iron is considered to be as follows.
The oxidation reaction of P, Cr and iron (Fe) are regarded to be as follows: ##EQU1## The numerical value for pressure indicated on the right side of each equation represents the equilibrium oxygen (O) partial pressure under the standard state at 1500.degree. C. for each substance. It will be learned from these data that Cr combines with oxygen far easier than P and Fe. This fact is one of the reasons that the dephosphorization of the molten Cr pig iron is extremely difficult in comparison with that of molten plain pig iron containing no Cr. That is to say, in the prior art processes, the intention to dephosphorize by oxidation resulted in oxidation of Cr only, and oxidation of P did not occur. Even if P is oxidized, Cr is oxidized far more. Also it has been learned that the produced oxide of Cr (referred to as Cr.sub.2 O.sub.3) impairs dephosphorizing power of the slag. It is understood that the formed Cr.sub.2 O.sub.3 acts as an acidic oxide and combines with P.sub.2 O.sub.5 -fixing materials and substantially reduces their P.sub.2 O.sub.5 -fixing ability. That is, in the case of the Cr pig iron, the fixation of formed P.sub.2 O.sub.5 is difficult, that is, the so-called rephosphorization becomes a serious problem.
Therefore, in order to carry out dephosphorization of the Gr pig iron, it is necessary to promote the reaction EQU 2P+5FeO.fwdarw.P.sub.2 O.sub.5 +5Fe (4)
and at the same time to control as much as possible the reaction EQU 2Cr+3FeO.fwdarw.Cr.sub.2 O.sub.3 +3Fe (5)
The known measures for oxidizing a molten iron bath as controlling oxidation of Cr therein are to reduce the partial pressure of CO of the atmosphere. Specifically speaking, it is known to reduce the pressure of the surrounding atmosphere or to contact a gaseous mixture of an oxidizing gas such as oxygen (O) and an inert gas such as argon (Ar) or nitrogen (N) with the molten iron bath.
Another means for dephosphorizing while controlling oxidation of Cr is to reduce the oxygen potential of the iron bath. The decrease in the oxygen potential of the iron bath can be achieved by increase in the silicon (Si) content in the bath. But it is not desirable because Si is oxidized to SiO.sub.2, which lowers basicity of the slag. In this respect, carbon (C) is oxidized to produce CO which has no influence on the slag property. Therefore increase in the C content of the bath is preferred.
According to the knowledge hitherto, as noted in Japanese Laying-Open Patent Publication No. 28511/78 quoted above, which relates to plain carbon steel, and foreseen from the above equation (1), it is thought that in order to promote oxidation of P, the oxygen potential of the iron bath should be raised. In the case of the Cr pig iron, however, it was quite unknown whether oxidation of P (dephosphorization) will satisfactorily occur or not, if the oxygen potential of the iron bath is lowered in order to control oxidation of Cr.
On the basis of the above-mentioned understanding, we tried dephosphorization of the Cr pig iron using the known dephosphorization slags under the conditions under which oxidation of Cr is controlled. But we could not succeed by merely controlling oxidation of Cr. Repeated varied experiments revealed that among alkali metal compounds, Li.sub.2 O and Li.sub.2 CO.sub.3 are especially effective for the purpose of dephosphorization. (Hereinafter in this specification we mean Li.sub.2 O and/or Li.sub.2 CO.sub.3 by the term "the Li compound".)
We carried out dephosphorization of the Cr pig iron using slags containing alkali metal compounds and analyzed the slag after dephosphorization by means of X-ray diffraction method. And we have found that in the presence of Cr.sub.2 O.sub.3, affinity of the slag containing the Li compound to P.sub.2 O.sub.5 is stronger than the slag containing compounds of K and Na.
Further we have learned that we have to improve the slag to which the Li compound is added in order to make the contained Li compound act effectively.
Thus we have devised a proper composition of slag in which the costly Li compound is effectively utilized, and thus succeeded in dephosphorization of the Cr pig iron by oxidizing refining, which can be carried out at temperatures not higher than 1600.degree. C. And also it has been revealed that denitrification can be simultaneously effected by this method.